The disclosure relates generally to combined cycle operation, and more particularly, to systems and methods for controlling drum water level, such as during operation of a heat recovery steam generation (HRSG) system for a combined cycle power plant.
Drum type boilers are the most commonly used boilers in combined cycle power plants. In the power generation industry, drum level trips due to ineffective drum level control response during transient operations are a primary cause of disruption in power generation and lead to a substantial loss of plant availability and revenue. With increasingly unpredictable loads and combination with less consistent renewable energy sources (e.g. wind and solar), HRSG systems must compensate for more variable demands, increasing transient operations.
Control of drum water level is a challenging problem due to complicated demands of two-phase flows, presence of waves, unknown heat and pressure disturbances, and load demands. Maintaining water level within limits is critical, as exceeding the limits will lead to trips or damage the equipment. A drop in water level will cause thermal fatigue in the drum. An increase in water level significantly increases the possibility of water droplets entering a superheater/steam turbine.
Conventional approaches to control of water level typically include actuation of a feed water control valve that supplies water to the drum in reaction to observed changes in level and steam flowrate. However, such approaches to control water level are challenging for drum type boilers, especially during transient operation conditions due to inverse response exhibited by such systems. The disturbance rejection responses of level controllers of existing systems may not be adequate to handle such pressure disturbances arising from bypass operations.
There is therefore a need for a more effective technique for controlling a liquid level in a vessel, such as a drum type boiler, especially during transient operating conditions.